Integrated circuits are utilized in a wide variety of applications. For example, integrated circuits are found within computer systems, mobile telephones, portable digital music players, and automobiles, to name a few. One of the components that can be fabricated as part of an integrated circuit is an operational amplifier circuit, which can receive an input electrical signal and output that signal with increased power. One desirable type of operational amplifier is one that can produce a high drive output current while operating with low voltages. However, it can be difficult to design such an operational amplifier.
For example, one conventional technique is to use an almost fully differential topology for the operational amplifier, which increases the voltage swing to drive a class A/B output stage. This type of topology involves a differential signal path all the way up to the output stage. However, it is difficult to make this circuit a rail-to-rail input because, usually, two parallel differential input pairs are required to achieve operability over the entire range between the two rails. Note that depending on the input common-mode voltage of the operational amplifier, only one of these pairs is on at one time. When these pairs are switched, there is a significant change in the direct current (DC) bias voltages inside the operational amplifier circuit. As such, a conventional common-mode feedback circuit is included as part of the operational amplifier to try and maintain the DC bias voltages during the switch. However, the inclusion of the conventional common-mode feedback circuit fails to prevent a huge change in bias voltages and allows a huge increase in supply current during the switch.
As such, it is desirable to address one or more of the above issues.